Multi-level output driver with adjustable pre-distortion capability

ABSTRACT

A PAM (Pulse Amplitude Modulation) modulator driver is configured to receive a PAM input signal having N input amplitude levels and provide a PAM output signal having N output amplitude levels, where N is an integer. The PAM modulator driver circuit configured to electrically adjust amplitude levels in the PAM output signal.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/365,787, filed on Jul. 22, 2016, entitled “Multi-Level OutputDriver With Adjustable Pre-Distortion Capability,” the disclosure ofwhich is hereby incorporated by reference in its entirety for allpurposes.

BACKGROUND OF THE INVENTION

Embodiments of the present invention is directed to methods andapparatus for a signal modulator driver. Some embodiments of theinvention are directed to methods and apparatus for realization of a PAM(Pulse Amplitude Modulation) modulator driver providing electricallyadjustable pre-distortion of the PAM output waveform.

Pulse-amplitude modulation (PAM) is widely used in modulating signaltransmission of digital data. For example, PAM4 (Pulse AmplitudeModulation-4) is a modulation technique in which four distinct pulseamplitudes are used to convey the information, as shown in FIG. 1. Fouramplitude levels can be represented by two binary bits 00, 01, 11, and10, respectively. Each pair of bits is called a symbol. When one of thefour amplitudes is transmitted in a symbol period T, there are two bitstransmitted in parallel, therefore the data rate is doubled. In otherwords, PAM-4 modulation is twice as bandwidth-efficient as conventionalbinary modulation.

Conventional PAM4 modulator driver architectures used commonly in theart utilize a highly linear wide band power amplifier to amplify a4-level PAM4 input signal and provide a PAM4 output signal withsufficient amplitude to drive an optical modulator in a communicationssystem with low harmonic distortion. The inventors have identified manydrawbacks in the conventional approaches. For example, the conventionalapproach suffers from high power dissipation due to the requirement ofproviding high linearity and low distortion of the input signal. It alsosuffers from PAM4 output eye closure, overshoot, and undershoot due tothe non-ideal amplitude and group delay response in the amplifiertransfer characteristic, as well as rise and fall time degradation dueto bandwidth limitations.

From the above, it is seen that techniques for improved PAM4 modulatordriver are highly desired.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to method and apparatus for a PAMmodulator driver. Some embodiments of the invention are directed tomethod and apparatus for realization of a PAM4 modulator driverproviding electrically adjustable pre-distortion of the PAM4 outputwaveform. The electrically adjustable pre-distortion can be used for thepurpose of counteracting non-linearity associated with the opticalmodulator in an optical communications system.

According to some embodiments of the present invention, a PAM4 (PulseAmplitude Modulation-4) modulator driver circuit includes aPAM4-to-unary decoder configured to receive a PAM4 data signal and toconvert the PAM4 data signal to a thermometer code signal, thethermometer code signal including three data bits. The PAM4 modulatordriver circuit also includes a unary-to-PAM4 encoding driver coupled tothe PAM4-to-unary decoder to receive the thermometer code signal, and toconvert the thermometer code signal to a PAM4 output signal. Theunary-to-PAM4 encoding driver includes three variable current sourcesand is configured to provide electrically adjustable signal levels inthe PAM4 output signal. Depending on the embodiment, the PAM4 outputsignal can be a differential signal or a single-ended signal.

In some embodiments of the PAM4 modulator driver circuit, theunary-to-PAM4 encoding driver includes a first circuit including a firstswitch and a first adjustable current source and configured to receive afirst data bit of the thermometer code signal, a second circuitincluding a second switch and a second adjustable current source andconfigured to receive a second data bit of the thermometer code signal,and a third circuit including a third switch and a third adjustablecurrent source and configured to receive a third data bit of thethermometer code signal. The unary-to-PAM4 encoding driver also has acurrent summing buffer coupled to the first circuit, the second circuit,and the third circuit. A load circuit is coupled to the current summingbuffer and configured to provide an output voltage signal based on a sumof current from each of the first, the second, and the third circuits.In some embodiments, the unary-to-PAM4 encoding driver also includesthree current control signals for adjusting, respectively, the firstadjustable current source, the second adjustable current source, and thethird adjustable current source.

In some embodiments of the PAM4 modulator driver circuit, the PAM4 datasignal includes two differential PAM4 data signals. In an embodiment,the PAM4-to-unary decoder includes a differential resistor laddercircuit to receive the two differential PAM4 data signals and to providemultiple internal signals. The PAM4-to-unary decoder also includes threedifferential comparators coupled to the differential resistor laddercircuit to receive the multiple internal signals, and the threedifferential comparators configured to provide the thermometer codesignal.

In some embodiments, the differential resistor ladder circuit includes afirst resistor ladder for receiving a first differential PAM4 signal.The first resistor ladder includes a first transistor, a first resistor,a second resistor, and a first current source coupled in series at,respectively, three internal nodes and configured to provide a firstgroup of three voltage signals. The differential resistor ladder circuitalso includes a second resistor ladder for receiving a seconddifferential PAM4 signal. The second resistor ladder including a secondtransistor, a third resistor, a fourth resistor, and a second currentsource coupled in series at, respectively, three internal nodes and isconfigured to provide a second group of three voltage signals.

In some embodiments, each of the three differential comparatorscomprises two input terminals for receiving a first signal from one ofthe first group of three voltage signals of the first resistor ladder,and a second signal from one of the second group of three voltagesignals of the second resistor ladder. Each differential comparator isconfigured to compare the first signal and the second signal to providedifferential output signals representing one of three data bits of thethermometer code signal.

According to some embodiments of the present invention, A PAM (PulseAmplitude Modulation) modulator driver circuit includes a PAM-to-unarydecoder configured to receive a PAM input signal and convert the PAMinput signal to a unary code signal. The PAM modulator driver circuitalso has a unitary-to-PAM encoding driver coupled to the PAM-to-unarydecoder and configured to convert the unary code signal to a PAM outputsignal. In some embodiments, the PAM modulator driver circuit isconfigured to electrically adjust amplitude levels in the PAM outputsignal.

In some embodiments, the PAM input signal has N amplitude levels, theunary code signal has N−1 data bits, and the PAM output signal has Namplitude levels, where N is an integer. The unitary-to-PAM encodingdriver includes N−1 current sources. Each of the current sources iscontrolled by one of the data bits in the unary code signal.

In some embodiments, the N−1 current sources are adjustable currentsources configured to provide current levels adjustable by a respectivecontrol signal. The adjustable current sources are configured toelectrically adjust amplitude levels in the PAM output signal.

In some embodiments, the PAM modulator driver circuit is a PAM4 (PulseAmplitude Modulation-4) modulator driver circuit. The PAM4 modulatordriver circuit includes a PAM4-to-unary decoder configured to receive aPAM4 data signal and to convert the PAM4 data signal to a thermometercode signal having three data bits. The PAM4 modulator driver circuitalso includes a unary-to-PAM4 encoding driver coupled to thePAM4-to-unary decoder to receive the thermometer code signal, and toconvert the thermometer code signal to a PAM4 output signal. Theunary-to-PAM4 encoding driver includes three variable current sourcesand is configured to provide electrically adjustable signal levels inthe PAM4 output signal.

According to some embodiments of the present invention, a PAM (PulseAmplitude Modulation) modulator driver is configured to receive a PAMinput signal having N input amplitude levels, where N is an integer, andprovide a PAM output signal having N output amplitude levels. The PAMmodulator driver includes a plurality of adjustable current sourcesconfigured to provide adjustable currents for electrically adjustingamplitude levels in the PAM output signal. In some embodiments of theabove PAM modulator driver, N=2^(K), where K is an integer.

In some embodiments, the PAM modulator driver includes a PAM-to-binarydecoder configured to receive a PAM input signal and to convert the PAMinput signal to a binary code signal, and a binary-to-PAM encodingdriver coupled to the PAM-to-binary decoder and configured to convertthe binary code signal to a PAM output signal. The PAM modulator driveris configured to provide electrically adjustable amplitude levels in thePAM output signal. In some embodiments, the plurality of adjustablecurrent sources is configured to provide currents having binary weightedamplitudes.

In some embodiments, the PAM modulator driver includes a PAM-to-unarydecoder configured to receive a PAM input signal and to convert the PAMinput signal to a unary code signal, and a unary-to-PAM encoding drivercoupled to the PAM-to-unary decoder and configured to convert the unarycode signal to a PAM output signal. The PAM modulator driver isconfigured to provide electrically adjustable amplitude levels in thePAM output signal. In some embodiments, the plurality of adjustablecurrent sources is configured to provide currents having equalamplitude.

According to some embodiments of the present invention, a decoder forconverting a multi-level signal to digital signal is provides. Eachdigital signal has a number of data bits, and each data bit representone of two data values. The multi-level signal to digital signal decoderincludes a resistor ladder circuit to receive the multi-level signal andto provide multiple internal signals. The multi-level signal to digitalsignal decoder also include a plurality of comparators coupled to theresistor ladder circuit to receive the multiple internal signals, and isconfigured to provide the digital signal. In some embodiments, theresistor ladder circuit includes a first transistor, a number ofresistors, and a first current source coupled in series at,respectively, multiple internal nodes and configured to provide multiplevoltage signals.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a waveform diagram illustrating an example of a PAM4 signalwaveform;

FIG. 2 is a simplified block diagram illustrating a system and method inwhich a PAM4 output waveform with large amplitude swing is generatedaccording to an embodiment of the present invention;

FIGS. 3A and 3B illustrate the conversion from a PAM4 signal to a unarycode signal;

FIG. 4 illustrates a block diagram of an example of the Unary-to-PAM4Encoding Driver shown in FIG. 2 according to an embodiment of thepresent invention;

FIG. 5 illustrates a block diagram of a Unary-to-PAM4 Encoding Drivergenerating a single-ended PAM4 (4-state) output signal according to anembodiment of the present invention;

FIG. 6 illustrates a circuit diagram of the single-ended Encoding Driveraccording to an embodiment of the present invention;

FIGS. 7A-7D are PAM4 eye diagrams illustrating pre-distortion adjustmentcharacteristics that can be generated on the PAM4 output signalaccording to an embodiment of the present invention;

FIGS. 8A-8D illustrate simulation waveforms showing the pre-distortioncharacter described above in connection with FIG. 7 according to anembodiment of the present invention;

FIG. 9 illustrates a block diagram of a PAM4-to-Unary Decoder in FIG. 2according to an embodiment of the present invention;

FIG. 10 is a circuit diagram illustrating one implementation of thedifferential resistor ladder in FIG. 9 according to an embodiment of thepresent invention; and

FIG. 11 is a circuit diagram illustrating one implementation of thedifferential comparator in FIG. 9 according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Some embodiments of the present invention are directed to method andapparatus for a PAM (Pulse Amplitude Modulation) modulator driverproviding signal amplification and electrically adjustablepre-distortion of the PAM output waveform.

In some embodiments, a PAM modulator driver is configured to receive aPAM input signal having N input amplitude levels, where N is an integer.The PAM modulator driver is also configured to provide a PAM outputsignal having N output amplitude levels. The PAM modulator driver caninclude a plurality of adjustable current sources configured to provideadjustable currents for electrically adjusting amplitude levels in thePAM output signal. The electrically adjustable pre-distortion can beused for the purpose of counteracting non-linearity associated with theoptical modulator in an optical communications system.

Embodiments of the present invention provide benefits over theaforementioned problems associated with conventional approaches throughthe novel combination of non-linear decoder and encoder blocks togenerate an output PAM waveform, which provides lower power dissipationthan conventional linear approaches. Embodiments of the invention offerfurther benefits of providing a limiting amplifier function whichprovides low output eye overshoot and undershoot characteristics as wellas fast rise and fall times. The embodiments further implement apre-distortion capability for the output waveform for the purpose ofcounteracting non-linearity associated with the optical modulator in anoptical communications system.

In some embodiments, the PAM modulator driver can include aPAM-to-binary decoder configured to receive a PAM input signal and toconvert the PAM input signal to a binary code signal. The PAM modulatordriver can also have a binary-to-PAM encoding driver coupled to thePAM-to-binary decoder and configured to convert the binary code signalto a PAM output signal. The PAM modulator driver is configured toprovide electrically adjustable amplitude levels in the PAM outputsignal.

In some alternative embodiments, the PAM modulator driver can include aPAM-to-unary decoder configured to receive a PAM input signal and toconvert the PAM input signal to a unary code signal. The PAM modulatordriver can also have a unary-to-PAM encoding driver coupled to thePAM-to-unary decoder and configured to convert the unary code signal toa PAM output signal. The PAM modulator driver is configured to provideelectrically adjustable amplitude levels in the PAM output signal. Inthese embodiments, the data bits in the unary code signal are used tocontrol the adjustable current sources for electrically adjustingamplitude levels in the PAM output signal.

In embodiments where the PAM signal is converted to binary code signals,the data bits in the binary code signal are used to control theadjustable current sources for electrically adjusting amplitude levelsin the PAM output signal. In embodiments where the PAM signal isconverted to unary code signal, the data bits in the unary code signalare used to control the adjustable current sources for electricallyadjusting amplitude levels in the PAM output signal. In the followingdescription, example implementations are illustrated using PAM4 signalsand unary code conversion. However, it is understood that the techniquescan be applicable to other PAM signals, such as PAM8, PAM16, etc., andto binary code conversions as well.

FIG. 2 is a simplified block diagram illustrating a system and method inwhich a PAM4 output waveform with large amplitude swing is generatedaccording to an embodiment of the present invention. As shown in FIG. 2,system 200 includes a PAM4-to-Unary Decoder 210 and a Unary-to-PAM4Encoding Driver 220. A PAM4 (4-state) input waveform 230 is firstdecomposed into three unary NRZ (Non-Return to Zero) 2-state waveformsU1, U2, and U3 (240) using thermometer coding in PAM4-to-Unary Decoder210. The resultant three unary NRZ waveforms 240 are then amplified andsummed to create a high-swing PAM4 output waveform 250 in Unary-to-PAM4Encoding Driver 220.

FIGS. 3A and 3B illustrate the conversion from a PAM4 signal to a unarycode signal. FIG. 3A shows that a PAM4 signal having four signalamplitude levels V0, V1, V2, and V3 are converted to unary code signalsU1, U2, and U3, in this case a thermometer code signal. It can be seenthat V0 is converted to 000, V1 is converted to 001, V2 is converted to011, and V3 is converted to 111. FIG. 3B shows an example with specificsignal levels. It can be seen in FIG. 3B that a PAM4 input signal Vin isa differential signal with 900 mV peak-to-peak amplitude range. Thefours signal levels in the PAM4 signal has signal amplitudes at −450 mV,−150 mV, +150 mV, and +450 mV. Three comparator thresholds are shown forU1, U2, and U3, respectively, at 300 mV, 0 mV, and −300 mV. FIG. 3B alsoshows that it can be seen that V0 is converted to 000, V1 is convertedto 001, V2 is converted to 011, and V3 is converted to 111.

FIG. 4 illustrates a block diagram of a Unary-to-PAM4 Encoding Driveraccording to an embodiment of the present invention. Unary-to-PAM4Encoding Driver 400 in FIG. 4 is an example driver that can be used asUnary-to-PAM4 Encoding Driver 220 shown in FIG. 2. In FIG. 4, a firstcircuit 401 includes a first switch 421 and a first adjustable currentsource 451 and is configured to receive a first data bit of thethermometer code signal U1. A second circuit 402 includes a secondswitch 422 and a second adjustable current source 452 and is configuredto receive a second data bit of the thermometer code signal U2. A thirdcircuit 403 includes a third switch 423 and a third adjustable currentsource 453 and is configured to receive a third data bit of thethermometer code signal U3. The unary-to-PAM4 encoding driver also has acurrent summing buffer 430 coupled to the first circuit, the secondcircuit, and the third circuit. A load circuit, including resistors 441and 442, is coupled to the current summing buffer 430 and is configuredto provide an output voltage signal 460 based on a sum of current fromeach of the first, the second, and the third circuits. In someembodiments, the unary-to-PAM4 encoding driver also includes threecurrent control signals 461, 462, and 463 for adjusting, respectively,the first adjustable current source, the second adjustable currentsource, and the third adjustable current source.

In FIG. 4, the inputs to Unary-to-PAM4 Encoding Driver 400 areessentially three NRZ (2-state) data signals labeled U1, U2, and U3(410). Adjustable current sources I1, I2, and I3 provide currents forcurrent switches 421, 422, and 423, which steer the current from one oftwo input current polarities (plus or minus) to a current summing buffer430 based on the digital switching of signals U1, U2, and U3,respectively, which then sums and buffers the currents of the threecurrent switches to two load resistors 441 and 442 (one for eachpolarity) connected to a DC supply voltage. The load resistors 441 and442 are used to convert currents into PAM4 output voltages 460. Each ofthe currents I1, I2, and I3 are independently electrically adjustable.Through the switching action of the current switches on currents I1, I2,and I3 steering those currents to the load resistors based uponindependent input data streams U1, U2, and U3, respectively, a 4-levelPAM4 output waveform is generated, as illustrated in FIGS. 3A and 3B.Through the adjustment of current sources I1, I2, and I3 by adjustmentcontrol signals 461, 462, and 463, the PAM4 output signal is amplifiedand various pre-distortion characteristics can be generated with respectto the PAM4 output signal that will be illustrated later.

FIG. 5 illustrates a block diagram of a Unary-to-PAM4 Encoding Drivergenerating a single-ended PAM4 (4-state) output signal according to anembodiment of the present invention. This configuration is similar tothe differential configuration previously described in FIG. 4, whichprovides differential output signal 460; only the differences will bedescribed here. In this configuration, a second DC voltage, DC Voltage 2(540), is used to source the current to the unused current switch outputpolarity for each of the current switches connected to I1, I2, and I3respectively. These currents represent the “zero state” for U1, U2, andU3 input data signals. In the “one state” of U1, U2, and U3 data,currents I1, I2, and I3, respectively, are summed and buffered on theload resistor 542. The load resistor 542 forms a single-ended PAM4output signal from these summed currents. One advantage of this topologyfor the generation of a single-ended PAM4 output signal is that DCVoltage 2 can be substantially lower than DC Voltage 1, resulting inlower power dissipation of this circuit.

FIG. 6 illustrates a circuit diagram of the single-ended Encoding Driveraccording to an embodiment of the present invention. In thisconfiguration, the current sources are realized using an NPN transistor621, 622, and 623 with adjustable reference voltages presented to thebase terminal denoted by VREF1, VREF2, and VREF3 in the diagram. Thecurrent switches are realized using a common-emitter differential pairof NPN transistors 631, 632, and 633. The current summing buffer isrealized using NPN transistors 641 in a common-base configuration, thusforming a cascode topology with the common-emitter NPN transistors ofthe current switches. Additionally, bias circuitry and bypasscapacitance is provided for the base terminals of the common-basecurrent summing buffer transistors.

FIGS. 7A-7D illustrate the pre-distortion characteristics that can begenerated on the PAM4 output signal through the use of the EncodingDriver circuit configuration and through the setting of currents I1, I2,and I3 according to embodiments of the present invention describedabove. The upper left diagram FIG. 7A illustrates that a PAM4 eye can begenerated with no pre-distortion (evenly spaced levels) through settingof current I1 equal to I2 equal to I3. The upper right diagram FIG. 7Billustrates a PAM4 eye with symmetrical pre-distortion through settingof current I1 equal to I3, and I2 to greater than I1. The lower leftdiagram FIG. 7C illustrates a PAM4 eye with symmetrical pre-distortionthrough setting of current I1 equal to I3, and I2 to less than I1. Thelower right diagram FIG. 7D illustrates a PAM4 eye with asymmetricalpre-distortion through setting of current I1 to greater than I2 togreater than I3.

FIGS. 8A-8D illustrate simulation waveforms illustrating thepre-distortion character described above in connection with FIGS. 7A-7Daccording to an embodiment of the present invention.

The Encoding Driver circuit illustrated can be used as an output stageof a multi-stage driver IC (an example Pre-Driver stage will beillustrated later), or as a standalone driver IC with the addition ofinput termination resistors and input biasing circuitry not shown butwell known to someone skilled in the art. Other permutations that fallwithin the present invention which can be accomplished by one skilled inthe art include using the opposite input signal polarity on one or moreinput signals, using FET transistor or a combination to bipolar and FETtransistors in the realization of the current switch, current source, orsumming buffer functions, using the same power supply for DC Voltage 1and 2, using a DAC controlled circuit for generation of VREF1-3, usingadditional current switch stages and reference currents in summation onthe common load resistor, using any combination of fixed or variablecurrent sources, to use an active load instead of a load resistor, usinga remote load resistor at an optical modulator or at a direct modulatedlaser diode, or using any combination of positive, negative, and groundpotentials to bias the driver, etc.

FIG. 9 illustrates a block diagram of a PAM4-to-Unary Decoder accordingto an embodiment of the present invention. As shown in FIG. 9,PAM4-to-Unary Decoder 900 is an example circuit that can be used asPAM4-to-Unary Decoder 210 in FIG. 2. PAM4-to-Unary Decoder 900 includesa differential resistor ladder 910 and multiple differential comparators920. The PAM4 (4-state) input data waveform 920 is decoded into threeunary NRZ (2-state) waveforms via thermometer coding using adifferential resistor ladder 910, which generates 6 outputs C, X, B, Y,A, and Z (3-levels for each polarity) 930, followed by 3 differentialcomparators 920, which generates three limiting NRZ thermometer outputwaveforms 960 including U1, U2, and U3.

FIG. 10 illustrates one implementation of the differential resistorladder according to an embodiment of the present invention. As shown inFIG. 10, differential resistor ladder 1000 is an example of differentialresistor ladder 910 of FIG. 9. In the embodiment of FIG. 10,differential resistor ladder circuit 1000 includes a first resistorladder 1010 and a second resistor ladder 1020. The first resistor ladder1010 is configured for receiving a first differential PAM4 signal, e.g., Vin_P, and includes a first transistor 1012, a first resistor 1014,a second resistor 1016, and a first current source 1018 coupled inseries at, respectively, three internal nodes and configured to providea first group of three voltage signals, A, B, and C. The second resistorladder 1020 is configured for receiving a second differential PAM4signal, e.g., Vin N, and includes a second transistor 1022, a thirdresistor 1024, a fourth resistor 1026, and a second current source 1028coupled in series at, respectively, three internal nodes and configuredto provide a second group of three voltage signals X, Y, and Z.

In the embodiment of FIG. 10, current sources 1018 and 1028 are fixedcurrent sources supplying a current I0, realized using NPN transistorswith an adjustable reference voltage presented to the base terminaldenoted by VREF. Resistor ladders having resistors 1014, 1016, 1024 and1028 with resistance values R set up fixed static DC voltage offsets ofI0*R for nodes A and B, B and C, as well as nodes X and Y, Y and Z whenno input signal is applied. When a PAM4 input waveform is applied, Vin_Pis buffered through the emitter follower 1012 and the voltages on nodesA, B, and C all get a delta-voltage value equal to Vin_P. Similarly, thenodes X, Y, Z get a delta-voltage value equal to Vin N since bothcurrent source I0 and resistor values R are fixed, and both areindependent of input voltage Vin. The comparators 920 in FIG. 9 then arewired such that the I0*R offsets create the proper positive or negativeoffsets for the correct comparator thresholds for generating U1, U2, andU3 comparator outputs based on the fixed amplitude differential PAM4input signal waveform as shown in the previous figure. The currents I0can be generated to be dependent on sheet resistance value such thatI0*R is a constant voltage value.

FIG. 11 illustrates one implementation of the differential comparatoraccording to an embodiment of the present invention. Differentialcomparator 1100 in FIG. 11 is an example of one of the differentialcomparators 920 of FIG. 9. As shown in FIG. 11, differential comparator1100 is configured to receive first and second input signals 1101 and1102 and to compare the voltage levels of the first signal and thesecond signal to provide differential output signals 1121 and 1122representing one of three data bits of the thermometer code signal. Asshown in FIG. 9, each differential comparator 920 has two inputterminals for coupling to receive two signals from differential resistorladder 910. A first signal is selected from one of the first group ofthree voltage signals A, B, and C from the differential resistor ladderas shown in FIG. 10, and a second signal is selected from one of thesecond group of three voltage signals X, Y, and Z as shown in FIG. 10.As shown in FIG. 9, a first differential comparator 920 receives signalsC and X, a second differential comparator 920 receives signals B and Y,and a third differential comparator 920 receives signals A and Z.

In the embodiment of FIG. 11, the current source 1112 is realized usingan NPN transistor with an adjustable reference voltage presented to thebase terminal denoted by VREF in the diagram. The comparing function isrealized using a common-emitter differential pair of NPN transistors1113 and 1114 followed by common-base NPN transistors 1116 and 1117,thus forming a cascode topology with the common-emitter transistors 1113and 1114 of the differential pair. Additionally, bias circuitry andbypass capacitance is provided for the base terminals of the common-basetransistors.

The decoder circuit illustrated can be used as an input stage of amulti-stage driver IC (an example Driver stage was illustrated earlier),or as a standalone driver IC. Other permutations that fall within thepresent invention which can be accomplished by one skilled in the artinclude to use a single-ended resistor ladder, to use FET transistor ora combination to bipolar and FET transistors in the realization of thefollower, ladder, current source, and differential pair, to use a DACcontrolled circuit for generation of VREF, to use any combination offixed or variable current sources, to use an active load instead of aload resistor, or to use any combination of positive, negative, andground potentials to bias the driver, etc.

Further, the description of the decoder circuit in connection to FIGS.9-11 are not limited to PMA input signals or unary or binary outputsignals. For example, according to some embodiments of the presentinvention, a decoder is provided for converting a multi-level signal todigital signal. Each digital signal has a number of data bits, and eachdata bit represent one of two data values. The multi-level signal todigital signal decoder includes a resistor ladder circuit to receive themulti-level signal and to provide multiple internal signals. Themulti-level signal to digital signal decoder also includes a pluralityof comparators coupled to the resistor ladder circuit to receive themultiple internal signals, and configured to provide the digital signal.In some embodiments, the resistor ladder circuit can include a firsttransistor, a number of resistors, and a first current source coupled inseries at, respectively, multiple internal nodes and configured toprovide multiple voltage signals. For example, the multi-level signalmay include N signal levels, where N is an integer. The first transistoris configured to receive the multi-level input signal. The resistorladder circuit includes N−2 resistors and N−1 internal nodes forproviding N−1 voltage signals.

It is understood that the examples and embodiments described herein arefor illustrative purposes only, and not as a limitation in the scope ofthe present invention in any way. Various modifications or changes inlight thereof will be suggested to persons skilled in the art withoutdeparting from the spirit of the present invention. Some of themodifications can include, for example, but are not limited to, the useof FET, CMOS, BiCMOS, or Bipolar transistors in the circuit diagramsshown, the use of single-ended signals instead of the differentialsignals shown, or the use of binary decoding/encoding instead of theunary decoding/encoding shown.

What is claimed is:
 1. A PAM4 (Pulse Amplitude Modulation-4) modulatordriver circuit, comprising: a PAM4-to-unary decoder configured toreceive a PAM4 data signal and to convert the PAM4 data signal to athermometer code signal, the thermometer code signal including threedata bits; and a unary-to-PAM4 encoding driver coupled to thePAM4-to-unary decoder to receive the thermometer code signal, and toconvert the thermometer code signal to a PAM4 output signal; wherein theunary-to-PAM4 encoding driver includes three variable current sourcesand is configured to provide electrically adjustable signal levels inthe PAM4 output signal.
 2. The circuit of claim 1, wherein theunary-to-PAM4 encoding driver comprises: a first circuit including afirst switch and a first adjustable current source and configured toreceive a first data bit of the thermometer code signal; a secondcircuit including a second switch and a second adjustable current sourceand configured to receive a second data bit of the thermometer codesignal; a third circuit including a third switch and a third adjustablecurrent source and configured to receive a third data bit of thethermometer code signal; a current summing buffer coupled to the firstcircuit, the second circuit, and the third circuit; and a load circuitcoupled to the current summing buffer and configured to provide anoutput signal based on a sum of current from each of the first, thesecond, and the third circuits.
 3. The circuit of claim 2, furthercomprising three current control signals for adjusting, respectively,the first adjustable current source, the second adjustable currentsource, and the third adjustable current source.
 4. The circuit of claim1, wherein the PAM4 data signal comprises two differential PAM4 datasignals.
 5. The circuit of claim 4, wherein the PAM4-to-unary decodercomprises: a differential resistor ladder circuit to receive the twodifferential PAM4 data signals and to provide multiple internal signals;and three differential comparators coupled to the differential resistorladder circuit to receive the multiple internal signals, and the threedifferential comparators configured to provide the thermometer codesignal.
 6. The circuit of claim 5, wherein the differential resistorladder circuit comprises: a first resistor ladder for receiving a firstdifferential PAM4 signal, the first resistor ladder including a firsttransistor, a first resistor, a second resistor, and a first currentsource coupled in series at, respectively, three internal nodes andconfigured to provide a first group of three voltage signals; a secondresistor ladder for receiving a second differential PAM4 signal, thesecond resistor ladder including a second transistor, a third resistor,a fourth resistor, and a second current source coupled in series at,respectively, three internal nodes and configured to provide a secondgroup of three voltage signals.
 7. The circuit of claim 6, wherein eachof the three differential comparators comprises two input terminalsconfigured to receive: a first signal from one of the first group ofthree voltage signals of the first resistor ladder; and a second signalfrom one of the second group of three voltage signals of the secondresistor ladder; wherein each differential comparator is configured tocompare the first signal and the second signal to provide differentialoutput signals representing one of three data bits of the thermometercode signal.
 8. The circuit of claim 1, wherein the PAM4 output signalis a differential signal.
 9. The circuit of claim 1, wherein PAM4 outputsignal is a single ended signal.
 10. A PAM (Pulse Amplitude Modulation)modulator driver circuit, comprising: a PAM-to-unary decoder configuredto receive a PAM input signal and convert the PAM input signal to aunary code signal; and a unitary-to-PAM encoding driver coupled to thePAM-to-unary decoder and configured to convert the unary code signal toa PAM output signal.
 11. The circuit of claim 10, wherein the PAMmodulator driver circuit is configured to electrically adjust amplitudelevels in the PAM output signal.
 12. The circuit of claim 10, whereinthe PAM input signal comprises N amplitude levels, the unary code signalcomprises N−1 data bits, and the PAM output signal comprises N amplitudelevels, where N is an integer; and wherein the unitary-to-PAM encodingdriver comprises N−1 current sources, each of the current sources beingcontrolled by one of the data bits in the unary code signal.
 13. Thecircuit of claim 12, wherein the N−1 current sources are adjustablecurrent sources configured to provide current levels adjustable by arespective control signal, the adjustable current sources beingconfigured to electrically adjust amplitude levels in the PAM outputsignal.
 14. The circuit of claim 10, wherein the PAM modulator drivercircuit is a PAM4 (Pulse Amplitude Modulation-4) modulator drivercircuit, the PAM4 modulator driver circuit comprising: a PAM4-to-unarydecoder configured to receive a PAM4 data signal and to convert the PAM4data signal to a thermometer code signal, the thermometer code signalincluding three data bits; and a unary-to-PAM4 encoding driver coupledto the PAM4-to-unary decoder to receive the thermometer code signal, andto convert the thermometer code signal to a PAM4 output signal; whereinthe unary-to-PAM4 encoding driver includes three variable currentsources and is configured to provide electrically adjustable signallevels in the PAM4 output signal.
 15. A PAM (Pulse Amplitude Modulation)modulator driver, configured to: receive a PAM input signal having Ninput amplitude levels, where N is an integer; and provide a PAM outputsignal having N output amplitude levels, wherein the PAM modulatordriver comprises a plurality of adjustable current sources configured toprovide adjustable currents for electrically adjusting amplitude levelsin the PAM output signal.
 16. The PAM modulator driver of claim 15,wherein N=2^(K), where K is an integer.
 17. The PAM modulator driver ofclaim 15, comprising: a PAM-to-binary decoder configured to receive aPAM input signal and to convert the PAM input signal to a binary codesignal; a binary-to-PAM encoding driver coupled to the PAM-to-binarydecoder and configured to convert the binary code signal to a PAM outputsignal; wherein the PAM modulator driver is configured to provideelectrically adjustable amplitude levels in the PAM output signal. 18.The PAM modulator driver of claim 17, wherein the plurality ofadjustable current sources is configured to provide currents havingbinary weighted amplitudes.
 19. The PAM modulator driver of claim 15,comprising: a PAM-to-unary decoder configured to receive a PAM inputsignal and to convert the PAM input signal to a unary code signal; and aunary-to-PAM encoding driver coupled to the PAM-to-unary decoder andconfigured to convert the unary code signal to a PAM output signal;wherein the PAM modulator driver is configured to provide electricallyadjustable amplitude levels in the PAM output signal.
 20. The PAMmodulator driver of claim 19, wherein the plurality of adjustablecurrent sources is configured to provide currents having equalamplitude.